1. Field of the Invention
The present invention relates to a method of metering a composite molding compound consisting of a resin and glass fibers or the like, to be applied to an injection molding mechanism, and a metering system for carrying out the same.
2. Description of the Prior Art
The compressibility of a composite molding compound consisting of a resin and glass fibers or the like, such as BMC (bulk molding compound) or SMC (sheet molding compound) is somewhat higher than that of a pure resin. Therefore, it is difficult to charge the accurate quantity of such a composite molding compound. Since glass fibers contained in a composite molding compound are entangled, injection molding machines of a screw type are rarely used for injecting a composite molding compound and injection molding machines of a plunger type are used in most cases for injecting a composite molding compound to avoid the dissociation and breakage of the entangled glass fibers.
FIG. 2 shows a prior art injection molding machine of a plunger type. This injection molding machine has an injection barrel 5, an injection plunger 6 slidably fitted in the injection chamber of the injection barrel 5, a stroke detecting mechanism 8 connected to the injection plunger 6, a compound stocker 3 containing a molding compound 4 and connected to the injection barrel 5, a metering cylinder 1 combined with the compound stocker 3 to feed the molding compound 4 contained in the compound stocker 3 into the injection barrel 5, and an adjustable-pressure relief valve 9 through which returning working fluid discharged from a back working chamber 7H of the injection cylinder 7, i.e., a working chamber of the injection cylinder 7 on the side of the cylinder head of the injection cylinder 7, is discharged.
In metering the molding compound 4 by the metering cylinder 1, the pressure of the returning working fluid discharged from the back working chamber 7H of the injection cylinder 7 is regulated by the adjustable-pressure relief valve 9 to apply a regulated back pressure to the injection plunger 6 so that the molding compound 4 is compressed during metering. Thus, the molding compound 4 can be relatively stably and correctly metered even if the compressibility of the molding compound 4 is relatively high.
In this prior art injection molding machine, in general, the ratio of the sectional area of the injection chamber of the injection barrel 5 to that of the injection cylinder 7 is 1/10 and usually the back pressure necessary for the compression metering of the molding compound 4 is in the range of 0 to 30 kg/cm.sup.2. Accordingly, the adjustable-pressure relief valve 9 is required to regulate the pressure of the working fluid in the very low pressure regulating range of 0 to 3 kg/cm.sup.2 to regulate the back pressure acting on the injection plunger 6.
Generally, the pressure regulating range for the adjustable-pressure relief valve 9 is 3 kg/cm.sup.2 or above. Therefore, it is very difficult for the adjustable-pressure relief valve 9 to regulate the pressure of the working fluid in such a low pressure regulating range and the adjustable-pressure relief valve 9 is unable to achieve reliable pressure regulating operation. Consequently, the metering cylinder 1 must be a large cylinder or a high-pressure cylinder. Furthermore, if the fluidity of the molding compound is very low, it is possible that the molding compound does not flow smoothly during metering to cause short metering.
Furthermore, since the prior art injection molding machine detects the completion of metering operation merely through the detection of the stroke of the injection plunger 6, it is unlikely that reliability in metering a compressible molding compound can be further enhanced.